Presence of ferromanganese nodules on floors of the oceans has been known for over a hundred years due to reports by the British ship Challenger of results of the first long oceanographic voyage. With development of undersea cameras, it was found that these nodules occur very widely through the oceans and are present in vast quantities. The nodules vary widely in size and configuration. Chemical composition is somewhat variable but characteristically they are composed of metal oxides believed to be deposited at a very slow rate together with variable amounts of (probably occluded) non-metallic components such as silica, carbonate minerals and the like.
The metallic components of the nodules are primarily manganese and iron oxide. The nodules are found to be composed of extremely fine grains of manganese and iron oxide minerals of which todorokite, birnesite and delta manganese dioxide have been identified as manganese components. The only iron mineral which has been recognized is geothite, FeOOH. The nodules also contain smaller amounts of a large number of other metals, of which copper, cobalt and nickel are particularly interesting as a source for supply of industrially important metals for which land sources are becoming scarce. The form in which these minor components are present has not been firmly identified. Studies of chemical reactivity suggest that the metals other than manganese and iron occur as integral parts of the iron and manganese minerals. Possible proposed mechanisms include substitution in crystal lattices, ion exchange and adsorption. Whatever their form, the nickel and copper appear to be primarily associated with the manganese matrix and the cobalt with the iron matrix. Although it is known that different regions of a nodule may be relatively rich in one or the other of these groups of associated metals, no physical method has been described for concentration of either the iron or manganese component.
In recognition of the approaching scarcity of land supplies for some of the important metals, attention has been directed to methods for recovering the enormous amounts of nickel, cobalt and copper known to be present in ferromanganese nodules which can be dredged from the bottoms of the seas. There have been studies conducted which indicate that pyrometallurgy is unlikely to be an economic approach to winning of these metals from the deep sea nodules. Consequently, current investigations are primarily concerned with hydrometallurgy as the more promising approach. It has been recognized that the manganese dioxide of the manganese minerals must be reduced to a form which provides water soluble salts, e.g., manganous ion. Such dissolution of the manganese matrix with concurrent dissolution of the iron matrix releases the contained desirable metals for solution in an aqueous medium from which metal values may be extracted by various techniques, such as ion exchange with water immiscible solutions of chelating agents. The dissolution step can be conducted to leave the bulk of non-metallic mineral matter undissolved, simplifying operation of the subsequent step for recovery of desired metals.
Previously known dissolution techniques generally involve high energy consumption and pose environmental problems in disposal of spent reagents and by-products of the process. Sulfur dioxide can reduce the manganese dioxide and has been the reagent employed for that purpose in a number of reported studies. Sulfur dioxide is applied under high pressure or high temperature or both and leaves a problem of disposal of the sulfuric acid and sulfates generated by the reaction. The high requirements for power to operate compressors and/or for heat make this operation very unattractive for use on vessels at sea. Even at land based installations, the process is expensive. In either event, the streams discharged from the system pose a serious problem of environmentally acceptable disposal.
Dissolution by acetic acid is among the methods studied by Brooke and Prosser, as reported at Inst. Min. Metall. Trans. Sec. C, 78, C 64 (1969).